The present invention relates to a semiconductor device operated in a high-frequency frequency band.
A field effect transistors (hereinafter abbreviated as FET) using compound semiconductor such as GaN have been widely put in practical use as a semiconductor device operating in microwave frequency band, because it has excellent high-frequency characteristics.
The FET has, for example, a structure described below. A GaN buffer layer is formed on a SiC substrate. An undoped AlGaN layer having an active layer is formed on the GaN buffer layer. A drain electrode and a source electrode, in which Ti, Al, Ni, and Au are laminated in this order, are formed on the undoped AlGaN layer to make an ohmic junction between the undoped AlGaN layer using a liftoff method. Gate electrode composed of metals Pt and Au, which are laminated in this order, or composed of metals Ni and Au, which are laminated in this order, is formed between the drain electrode and the source electrode to make a Schottky junction with the undoped AlGaN layer also using the liftoff method.
However, adherence between GaN layer and Pt or between GaN layer and Ni is not good, because reactivity between GaN layer and Pt or between GaN layer and Ni are not good in the FETs. Therefore, there has been a problem that gate electrode formed by the liftoff method is peeled of f easily.
A method is known for forming gate electrode with a vapor deposition at a speed of about 2 to 20 Angstrom/sec, thereby improving the adherence between the gate electrode and GaN layer in order to prevent the defect described above (Japanese Unexamined Patent Application Publication No. H10-178189).
However, the method descried above is not sufficient to make good adherence when the gate length is decreased for a higher performance FET, because the junction area between the GaN layer and the gate electrode is decreased.
Therefore, one of the objects of the present invention is to provide a semiconductor device which prevents gate electrodes from peeling off.